In the human receptor tyrosine kinase superfamily, the ERBB family comprises four members: ERBB1 (epidermal growth factor receptor or EGFR), ERBB2 (HER2), ERBB3 (HER3), and ERBB4 (HER4). The ERBB receptors share an overall similar structure with a ligand-binding ectodomain, a single transmembrane domain, and an intracellular kinase domain, which is active in ERBB1, HER2 and ERBB4, but defective in ERBB3. A diverse array of ligands has been identified for the ectodomains of ERBB1, ERBB3, and ERBB4, but not HER2. Ligand binding induces conformational change in receptors to form homo- and hetero-dimerization. Without ligand binding, the extracellular domain of HER2 is already fixed in a conformation that resembles the other ligand-activated ERBB members, making it a preferred dimerization partner for other ligand-bound ERBBs. The dimerized receptors activate the intrinsic kinase activity, leading to phosphorylation of tyrosines at cytoplasmic tails. The ERBB receptors differ in kinase potency, phosphorylation sites, and substrate specificity. The phosphorylated tyrosines serve as the docking sites to recruit downstream effectors and activate multiple cascades of intracellular signaling pathways, including the anti-apoptotic/survival PI3K/AKT and the mitogenic RAS/RAF/MEK/ERK pathways. In normal cells, the activity of ERBB receptors is under tight control to regulate various cellular processes, such as growth, proliferation, development and differentiation, survival and apoptosis, cell shape and adhesion, migration, and angiogenesis. Yarden et al., Nat. Rev. Mol. Cell. Biol. 2001, 2, 127-137; Hynes et al., Nat. Rev. Cancer 2005, 5, 341-354.
As a major proliferation and survival engine for cells, constitutive activation of ERBB receptors, particularly ERBB1 and HER2, is oncogenic and can be a strong driver for tumorigenesis in cultured cells and animal models. In addition, the activated receptors accelerate cancer development by promoting tumor angiogenesis and metastasis. Persistent activation can result from overexpression of the receptors, production of excessive ligands, or generation of activating mutations in the ectodomains and kinase domains of the receptors. Yarden et al., Nat. Rev. Mol. Cell. Biol. 2001, 2, 127-137. In humans, genetic alterations in ERBB genes and other genes that lead to similar deregulation of ERBB receptors are frequently identified in majority of carcinomas, such as lung, breast, colon, prostate, brain, head and neck, oesophagus, ovary, cervix, bladder, stomach, and endometrium cancer. The aberrant activation of ERBB receptors is in general an adverse prognostic indicator for higher recurrence rate and shorter survival time. Nicholson et al., Eur. J. Cancer 2001, 37, 9-15; Slamon et al., Science 1997, 235, 177-182.
Given the compelling association of activation of ERBB receptors with human cancers, ERBB1 and HER2 are among the kinase targets for drug development, aiming to tame signaling transduction pathways for cancer treatment. To reverse the abnormal activity of ERBB receptors in tumors, monoclonal antibodies targeting the extracellular domains of ERBB1 and HER2 and small molecule chemicals inhibiting the intracellular kinase domains have been developed.
The monoclonal antibody drugs attack the ERBB receptors with high specificity and attenuate ERBB-mediated signaling by prevention of ligand binding and receptor dimerization, elimination receptors from cell surface through endocytosis, inhibition of shedding of extracellular domain, and activation of immune system. Hudis, N. Engl. J. Med. 2007, 357, 39-51. Cetuximab and panitumumab, two anti-ERBB1 antibodies, have shown improvement in response rate and the rate of progression-free survival in the treatment of metastatic colon cancer either as monotherapy or in combination with chemotherapies. In addition, cetuximab has also been approved for the treatment of locally advanced, unresectable or metastatic squamous cell carcinoma of the head and neck. Ciardiello et al., N. Engl. J. Med. 2008, 358, 1160-1174. Anti-HER2 antibody trastuzumab binds to the domain IV of the HER2 receptor at the juxtamembrane position. In clinical development, trastuzumab has demonstrated increased overall survival rate in early- and metastatic-stage breast cancer patients with tumors showing IHC 3+ HER2 overexpression or FISH gene amplification ratio of at least 2.0. Using the same criteria for patient selection, pertuzumab, which binds to a distinct epitope at the domain II of the HER2 receptor, has been found to further increase the complete response rate by addition to trastuzumab and docetaxel regimen as a neoadjuvant treatment for patients with locally advanced, early-stage breast cancer. Gradishar, N. Engl. J. Med. 2012, 366, 176-178.
Development of small-molecule ERBB1 kinase inhibitors (ERBB1Is) has become an evolving paradigm for using cancer genomics to guide targeted drug development and treatment. Gefinitib and erlotinib, the first two ERBB1I drugs, are reversible ATP mimetic inhibitors that bind to the wild-type ERBB1 catalytic domain to inhibit tyrosine kinase activity. In unselected patients of non-small cell lung cancer (NSCLC) or pancreatic cancer, only erlotinib has demonstrated clinical benefit by modestly increasing overall survival. Ciardiello et al., N. Engl. J. Med. 2008, 358, 1160-1174. In a subset of NSCLC patients that harbor activating mutations within ERBB1 tyrosine kinase domain, both gefitinib and erlotinib treatments are highly sensitive and can achieve lasting efficacy as monotherapy. These drug-responding mutations are mostly in-frame deletions nested around Leu-Arg-Glu-Ala from position 747 to 750 in ERBB1 exon 19, or a leucine to arginine substitution at position 858 (L858R) in exon 21.
However, the initial response to erlotinib or gefitinib relapses in 10-14 months by developing resistant mutations in tumors. Among them, a T790M gate-keeper point mutation in the exon 20 of ERBB1, which poses a steric interference to drug binding, is found in over 50% of acquired resistant tumors. To overcome the resistance from T790M mutation and confer sustained ERBB1 inhibition, the second-generation ERBB1Is have been developed, some of them are irreversible ERBB1 and HER2 dual inhibitors. The irreversible compounds overcome the kinase binding hindrance from T790M mutation by better fitting into the mutated binding pocket and forming covalent bond with the protein amino acid residues. Additionally, irreversible ERBB1Is appear to cause slower acquired resistance to the treatment than reversible inhibitors. Sharma et al., Nat. Rev. Cancer 2007, 7, 169-181.
In preclinical testing, afatinib, a second-generation ERBB1I, inhibited the growth of NSCLC HCC827 cells, which harbor the sensitive exon 19 deletion, and is 50-fold more potent than erlotinib in inhibition of growth of NSCLC H1975 cells, which has a T790M mutation in-cis with the L858R mutation. However, afatinib also inhibits A431 cells, whose growth is driven by a wild-type ERBB1, 100-fold more potent than H1975. The difference in potencies portends that, in cancer patients, the compound could inhibit wild-type ERBB1 completely before it reaches sufficient blood level for pharmacological effect on T790M mutant ERBB1. Since wild-type ERBB1 inhibition has been reported to cause dose-limiting toxicity in virtually all previously ERBB1I drugs, the preferential inhibition of wild-type ERBB1 over resistant mutant pose a potential challenge for afatinib to achieve high enough dose for T790M mutant inhibition. Consistent with the preclinical discovery, clinical development has found afatinib only showed equivalent efficacy to erlotinib or gefitinib in patients with sensitive mutations, but failed to demonstrate statistically meaningful superiority to erlotinib and gefitinib in treating patients with acquired T790M resistant mutation even at the maximum tolerated dose. Langer, J. Clin. Oncol. 2013, 31, 3303-3330. Thus, there is a clear and unmet need to develop effective therapeutics for treating a proliferative disease, especially drug-resistant cancer.